This invention relates to flowmeter instruments used in industrial applications, and more particularly, to an electronic flowmeter and preamplifier used with the flowmeter to deliver an electrical output signal having a value proportional to a measured flowrate. The flowmeter employs a scaling factor to take into account various operational factors of the flowmeter and the environment in which the fluid flow is measured, and both the timing and frequency of the generation of elements of the output signal are controlled.
Fluid flowmeters are used in a variety of applications. Some of the meters used; e.g., `PD`, turbine, and oval gear type meters, are mechanical meters. Others, e.g., magnetic, Coriolis, and vortex meters, are basically electronic flowmeters. These meters all provide an output signal which is a digital output. That is, the output comprises a train or stream of individual electrical pulses which together comprise the output signal. The pulses have signal characteristics which are a function of (proportional to) some measured unit of flow. Preferably, there is a linear relationship between the measured flowrate of a fluid and the digital signal output. This relationship is commonly referred to in the art as the K-factor and is usually ascertained during factory calibration of the meter. The value of the K-factor is stamped or otherwise recorded on the flowmeter. When the flowmeter output signal is provided to secondary instrumentation (electronic recording equipment, flow controllers, etc.) the gross flowrate of the fluid and a gross volume of flow can be determined using the K-factor.
In addition, other performance determinations can be made. One such determination is the net flowrate of the fluid. This value corrects for temperature and pressure effects on the fluid being metered; e.g., growth or shrinkage. A mass flowrate value can also be inferred if the density of the fluid is known or can be determined. Because of the various uses to which the flowrate information is used, it is extremely important that this information be accurate. This is particularly true in "custody-transfer" situations where a product is being conveyed between a buyer and a seller and the ultimate purchase price is based upon the measured quantity of fluid delivered through a pipeline or other means of conveyance. Such transfers are often governed by state or federal regulations and the flowmetering equipment used must further satisfy weight and measures standards imposed by these authorities.
A basic limitation of flowmeters is the minimum and maximum fluid flowrates over which they can accurately measure. The K-factor referred to above is generally specified for a particular flowrate/output pulse rate relationship. It is therefore accurate at that flowrate, but less so as the measured flowrates approach or exceed the minimum or maximum flowrates the meter can accommodate. In addition, the dynamics of the fluid flow effect the accuracy of the K-factor. That is, if the flow is turbulent rather than laminar, the accuracy of the K-factor is diminished.
Besides this calibration factor which is incorporated into the flowmeter readings, other factors must be considered as well. Another calibration factor is based upon the specific field conditions and wear to which the meter is or has been subjected, and the aging of the flowmeter components. This meter factor is developed during periodic field calibrations (proving) of the meter which take place after the meter is in use. This factor is used to "trim" the meter readings when an output pulse train from the meter is received by other equipment. A prover transports an accurate quantity of fluid through a flowmeter under certain test conditions (usually a steady flow rate), and an associated electronics measures the number of pulses emitted by the meter under test. Small volume (compact) provers go farther. These measure fractional portions of a meter pulse. This is done using a technique referred to as "dual-chronometry", and the technique maximizes the resolution of a calculated meter-factor.
Certain metering equipment now available utilizes multiple meter factors for use at different flowrates. For measured flowrates intermediate to those for which the factors are developed, the value of the factors is interpolated.
With mechanical flowmeters, other limitations must also be considered with respect to reading accuracy. For example, the K-factor relationship is good only for full or multiple revolutions of the mechanical metering element. There is a modulation effect produced by the gearing mechanism (such as occurs in oval gear flowmeters, for example). During uniform or steady-state flow conditions, this results in a repeatable pattern of pulses comprising an electrical output signal from the flowmeter. For low flow or unsteady flow situations, this condition causes non-linearity in the measurement which is, at times, unacceptable. And, current electronic systems are incapable of accommodating this variability in controlling the timing of individual pulses comprising a digital output of the flowmeter; this, even though the condition is a repetitive one.